A liquid crystal display device is widely used for the display of personal computers, televisions and so forth. This device utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal compound. The operating modes of this liquid crystal display device are known, such as a PC (phase change) mode, a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a BTN (bistable twisted nematic) mode, an ECB (electrically controlled birefringence) mode, an OCB (optically compensated bend) mode, an IPS (in-plane switching) mode, a VA (vertical alignment) mode and a PSA (polymer sustained alignment).
Among these, operating modes such as an ECB mode, an IPS mode and a VA mode utilize homeotropic orientation of liquid crystal molecules. It is known that a limited viewing angle, which is a disadvantage of a conventional display mode such as a TN mode and a STN mode, can be improved especially by means of the IPS mode and the VA mode.
A liquid crystal composition having negative dielectric anisotropy is mainly used for a liquid crystal display device operated by the IPS mode or the VA mode. It is desirable that a liquid crystal compound included in this composition should have physical properties shown in the following items (1) to (8), in order to further improve the characteristics of the liquid crystal display device.                (1) a high stability to heat, light or the like,        (2) a high clearing point,        (3) a low minimum temperature of a liquid crystal phase,        (4) a small viscosity (η),        (5) a suitable optical anisotropy (Δn),        (6) a large negative dielectric anisotropy (Δ∈),        (7) a suitable elastic constant (K33: bend elastic constant), and        (8) an excellent compatibility with other liquid crystal compounds.        
The effect of the physical properties of the liquid crystal compound on the characteristics of the device is as follows. A compound having a high stability to heat, light or the like, as described item (1), increases the voltage holding ratio of the device, as a result of which the service life of the device is increased. A compound having a high clearing point, as described in items (2), increases the temperature range in which the device can be used. A compound having a low minimum temperature of a liquid crystal phase such as a nematic phase and a smectic phase, especially of a nematic phase, as described in item (3), also increases the temperature range in which the device can be used. A compound having a small viscosity, as described in item (4), decreases the response time of the device.
A compound having a suitable optical anisotropy, as described in item (5), improves the contrast of the device. A compound having a large optical anisotropy or a small optical anisotropy, that is to say a suitable optical anisotropy, is necessary according to the design of the device. A compound having a large optical anisotropy is suitable when the response time is decreased by decreasing the cell gap of the device. A compound having a large negative dielectric anisotropy, as described in item (6), decreases the threshold voltage of the device, as a result of which the electric power consumption of the device is decreased.
In regard to the item (7), a compound having a large elastic constant decreases the response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Thus, a suitable elastic constant is necessary according to the characteristics that should be improved. A compound having an excellent compatibility with other liquid crystal compounds is desirable as described in item (8). This is because the physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
A variety of liquid crystal compounds having a large dielectric anisotropy have been prepared until now. This is because excellent physical properties which are not possessed by conventional compounds are expected. This is because a new compound is expected to possess a suitable balance between two physical properties which are required for the preparation of a liquid crystal composition. The compound (A) having bicyclo[3.3.0]octane-2,6-diyl has been reported (Patent document No. 1). This compound has positive dielectric anisotropy. However, a compound having bicyclo[3.3.0]octane-3,7-diyl and negative dielectric anisotropy is not known. In view of the circumstances, some new compounds are expected to have excellent properties.
